Styrene is an important monomer used in the manufacture of many plastics and is commonly produced in a two-step process. In the first step, benzene is alkylated with an ethylating agent, such as ethylene, over a molecular sieve catalyst to form ethylbenzene (EB) and polyethylbenzenes (PEBs), the latter comprising diethylbenzenes (DEBs), triethylbenzenes (TEBs), and heavier aromatic compounds. To maximize EB formation, the light PEBs (DEBs and TEBs) are usually transalkylated with benzene, again over a molecular sieve catalyst to form more EB. Both the alkylation and transalkylation effluents flow to a distillation train, which recovers unreacted benzene, EB, and the light PEBs as distillates in three distillation columns in series.
In the second step, the EB is dehydrogenated to styrene over an iron catalyst in the presence of steam, which supplies the sensible heat needed for the endothermic reaction and which reacts with coke deposited on the catalyst to form carbon oxides, mainly carbon dioxide. Benzene and toluene are by-products of the dehydrogenation reaction and so the crude styrene effluent leaving the reaction section of styrene plant comprises benzene, toluene, unreacted EB, styrene monomer, steam and small amounts of heavier aromatic compounds. Crude styrene condenses along with the steam in a final reactor effluent cooling step, forming separate aqueous and aromatic hydrocarbon phases, which are separated to recover an aromatic hydrocarbon phase saturated with water.
Acidity due to carbon dioxide dissolved in the aqueous phase of the crude styrene effluent can cause corrosion in downstream equipment and can hinder the separation of the organic and aqueous phases. Thus, in most plants, a corrosion inhibitor (a solution containing one or more amines or other basic nitrogen compounds) is added upstream of the condensation step to control corrosion. These compounds tend to partition with the aqueous phase in the separator. However, although these compounds typically boil at a higher temperature than the styrene monomer, they may break down to lighter nitrogen-containing compounds in the dehydrogenation reactor if a portion of the aqueous phase is used to generate the steam required in dehydrogenation.
After recovery of the crude styrene, the benzene/toluene (“B/T”) fraction is separated from ethylbenzene and heavier aromatics by distillation. The B/T fraction may also be recovered from an ethylbenzene-and-lighter fraction distilled from crude styrene in a preceding distillation column. In either configuration, water, CO2, and any light nitrogen-containing compounds present in the crude styrene effluent will distill with the B/T fraction, and a portion of these compounds will be present in B/T byproduct. Water and CO2 are present in the column overhead, again raising corrosion issues, so some plants inject additional corrosion inhibitor into the overhead. A separate water phase forms in the overhead condensing equipment and, although most of the amine injected will partition with the water phase, some will be present in the B/T byproduct.
As a result, the B/T fraction recovered from the dehydrogenation effluent typically contains high levels (in excess of 2 ppm) of basic nitrogen impurities, typically 10 to 100 times the concentration of nitrogen-containing compounds in the fresh benzene feed to the alkylation process. Although it would appear to be desirable to recycle at least the benzene in the B/T fraction to the alkylation reactor, the high concentration of nitrogen impurities in such a benzene recycle stream presents a serious challenge. Thus the active acid sites on the molecular sieve catalyst used in the alkylation reactor are titrated by basic compounds, such as the nitrogen impurities in the benzene byproduct, reducing the activity of the catalyst. In fact, acidic adsorbents are typically used to reduce even the relatively low levels of nitrogenous impurities present in the fresh benzene feed before this is allowed to contact the alkylation catalyst. There has therefore been significant disincentive in the art against recycling the benzene byproduct from the styrene plant back to the EB alkylation reactor.
Thus, U.S. Published Patent Application No. 2005/0288539 discloses a process for producing styrene in which the by-product benzene recovered from the EB dehydrogenation reaction is recycled to the transalkylation reactor rather than the alkylation reactor. The purported advantages of this process are that the adverse effect of this recycle stream is less on the transalkylation catalyst than on the alkylation catalyst and the transalkylation catalyst typically costs less than, and is cheaper to replace, than the alkylation catalyst.
U.S. Published Patent Application No. 2009/0149685 discloses a method for reducing alkylation catalyst poisoning, wherein the method comprises providing a dehydrogenation system including a dehydrogenation reactor and a separation system, wherein the separation system includes a first column and a second column, introducing an alkyl aromatic hydrocarbon into the dehydrogenation reactor, contacting the alkyl aromatic hydrocarbon with a dehydrogenation catalyst disposed within the dehydrogenation reactor to form a dehydrogenation output stream comprising a vinyl aromatic hydrocarbon, passing at least a portion of the dehydrogenation output stream to first column, recovering a first overhead fraction including benzene and a first bottoms fraction from the first column, passing at least a portion of the benzene from the first column to an alkylation system including an alkylation catalyst, passing the first bottoms fraction from the first column to the second column, recovering a second overhead fraction and a second bottoms fraction from the second column, withdrawing offtest from effluent streams selected from the dehydrogenation output stream, the first bottoms fraction, the second bottoms fraction and combinations thereof to form withdrawn offtest and introducing the withdrawn offtest into the separation system downstream from the first column.
According to the present invention, it has now been found that, by passage of at least the benzene by-product of a styrene plant through a dedicated adsorbent bed comprising at least one of acidic clay, alumina, an acidic ion exchange resin and an acidic molecular sieve, poisons, such as basic nitrogen compounds, can be effectively removed from the benzene by-product. The resultant purified benzene stream has a sufficiently low concentration of (typically less than 50 ppbw) of basic nitrogen compounds that the stream can be advantageously recycled to the EB alkylation and/or transalkylation reactor.